All-Optical Amplifiers Enable 20,000-km Link

Tackling two fundamental problems at the same time, scientists at Kailight Photonics Ltd. in Rehovot, Israel, and Bell Labs in Holmdel, N.J., have developed an all-optical wavelength converter that also amplifies, reshapes and retimes the pulses in a fiber optic telecom link. The device was described in a technical paper at the Optical Fiber Conference (OFC) in Los Angeles on Feb. 26, and the company displayed it on the exhibition floor.

Most fiber optic telecom systems depend upon stimulated emission in erbium-doped fiber amplifiers (EDFAs) to boost the pulse stream every 100 km or so along the link. But after repeated amplification, the pulses become noisy, distorted and jittery. To correct these problems, the optical pulses are converted to electrical pulses by a photodiode and are subsequently amplified, reshaped and retimed. The resulting clean electrical pulses then drive a laser, which converts them to optical pulses in the outgoing fiber. Wavelength conversion of the signal takes place if the laser transmits a different wavelength from the one detected by the photodiode, and it allows the clean signal to go into an unused channel in the outgoing fiber.

But the optical-to-electrical-to-optical (OEO) conversion process is awkward, inefficient and expensive. Engineers have long sought techniques that would enable an all-optical network capable of transmitting a signal tens of thousands of kilometers with no need for OEO conversion. The new device -- called the tunable, all-optical signal regenerator (TASR) -- accomplishes both wavelength conversion and signal amplification/enhancement optically.

Figure 1. The information on the incoming signal is transferred to the CW beam, resulting in an amplified, reshaped and, in some cases, retimed output pulse stream.The concept of the TASR is illustrated in Figure 1. The incoming signal enters port one of an optical circulator, emerges from port two and propagates through a semiconductor optical amplifier (SOA) in a direction opposite to an incoming CW laser beam. The interaction between the signal pulse and the CW beam is key to the device's operation. The modified CW beam enters the circulator's port two and emerges from port three. After it's filtered, it is an amplified, enhanced and wavelength-shifted version of the original, incoming signal.

To understand how the signal information is transferred to the CW laser beam, first imagine that a "zero" signal enters the circulator and passes through the SOA. The amplifier is unchanged, and the CW beam passing through it will be amplified, but its frequency will be unchanged. The output filter has a very sharp frequency response that cuts off immediately above the frequency of the CW laser, so no light emerges from the device when a "zero" enters.

Now imagine that a "one" signal passes through the SOA. It is amplified and, in the process, depletes the population inversion in the amplifier. The CW laser beam passing through at the same time is frequency-chirped because the population inversion is instantaneously changing. When this chirped light reaches the filter, its frequency has changed enough so that it is transmitted. Thus, a pulse of light emerges from the device in response to a "one" input signal.

The pulse of light has been wavelength-converted because the CW laser is a different wavelength from the incoming system. And because the amplifier is operating near saturation, the amplitude of the pulse that emerges is almost completely independent of the amplitude of the incoming signal pulse. In this case, the device has performed "2R" -- reamplification and reshaping -- amplification.Electronic amplifiers typically perform "3R" amplification: retiming the pulses -- that is, removing temporal jitter from the pulse stream -- as well as reamplifying and reshaping them. The TASR also is capable of 3R amplification if the CW laser is gated at the bit rate of the incoming signal, as indicated by the waveform at the bottom left of Figure 1. The concept also could be employed with a multichannel WDM signal if a periodic filter were placed on the output (Figure 2).

Figure 2. The concept is suitable for amplifying, enhancing and converting the wavelength of multiple channels of a WDM signal. TL = tunable laser; SOA = semiconductor optical amplifier.The researchers demonstrated the TASR's regenerative performance in a recirculating loop, with one of the devices placed every 400 km. The experimental setup consisted of four 100-km spans of TrueWave Reduced Slope nonzero-dispersion fiber and of appropriate lengths of dispersion-compensating fiber. EDFAs and backward-pumped Raman amplifiers offset the 21-dB span losses. For the return-to-zero transmitter, an electroabsorption modulator served as a pulse carver, and a LiNbO3 modulator encoded the 10-Gb/s data onto the signal.

Figure 3. A 10-Gb/s signal was transmitted through 20,000 km of fiber while maintaining a bit error rate of less than 10–9.The results of the demonstration are shown in Figure 3. The signal degenerated to a bit error rate of greater than 10–9 after traveling through 10 loops (4000 km) of the test fiber. When a TASR was added to the loop, however, the signal maintained a bit error rate of less than 10–9 after propagation through 50 loops (20,000 km).